1. Field of the Invention
The present invention relates to an image signal processing apparatus, which can be suitably used in an optical image reading apparatus of a facsimile apparatus, an image scanner, a copying apparatus and so on.
2. Description of the Related Arts
In such an optical image reading apparatus, a reflected light from an illuminated original is detected by an image reading sensor, and its sensed image signal is inputted to the image signal processing apparatus so as to form an image data corresponding to the original image. In the image signal processing apparatus, the MTF (Modulation Transfer Function) correction method may be employed so as to make the image data more precise with respect to the original image. FIG. 1 are schematic views to explain the MTF correction method.
In FIG. 1a, there are indicated picture elements P1 to P5, while there are indicated outputs N1 to N5 of the corresponding picture elements P1 to P5. Supposing that the picture element P1 is observed, the output N1 of the picture element of interest P1 is influenced by leaking out components from the peripheral picture elements P2 to P5 disposed adjacent in up-and-down and left-right directions with respect to the central picture element P1, and also by a leaking-out component from the picture element P1.
Accordingly, the relationship between the output N1 of the picture element of interest P1 and a true value N0, which is an ideal value corresponding to the real light and shade of the original image at the picture element of interest P1, is established as the following expression (1), EQU N1=N0-4K*N0+K(N2+N3+N4+N5) (1)
wherein, K represents the correction coefficient which value is peculiarly determined by the relevant optical system.
Thus, the true value N0 can be expressed by the following expression (2) from the expression (1). EQU N0={N1-K(N2+N3+N4+N5)}/(1-4K) (2)
In the aforementioned image signal processing apparatus, the MTF correction is performed as for the output N1 according to this expression (2), and thus corrected output is assumed as the true value N0 and utilized in following image signal processes.
The mutual relationship between the correction amount of the MTF correction method and the image signal level is expressed by the following expression (3), EQU N0-N1=[{N1-K(N2+N3+N4+N5)}/(1-4K)]-N1 (3)
wherein, (N0-N1) represents the MTF correction amount.
Thus, the following expression (4) can be obtained from the expression (3), EQU N0-N1={-K(N2+N3+N4+N5-4N1)}/(1-4K) (4)
wherein, the correction coefficient K is in the range as the following expression (5). EQU 0&lt;K&lt;1 (5)
Accordingly, the larger is the difference between the output N1 of the central picture element P1 and the outputs N2 to N5 of the peripheral picture elements, the larger is the correction amount (N0-N1).
FIG. 2 is a graph to explain the MTF. As shown in FIG. 2, supposing that an ideal output difference between a white picture element and a black picture element is V2, and that a detected output difference obtained at the time of the actual detection is V1, the MTF can be expressed by the following expression (6). EQU MTF=V1/V2 (6)
FIG. 3 shows the relationship between the MTF and a spatial frequency of the original image. As shown in FIG. 3, the MTF decreases as the spatial frequency increases. Thus, as the spatial frequency increases, the output difference between the central picture element (P1 in FIG. 1a) and the peripheral picture elements (P2 to P5 in FIG. 1a), decreases.
Accordingly, the purpose of the output correction performed in the optical image reading apparatuses, is in fact to compensate the diminished component due to the increase of the spatial frequency. More specifically, the purpose is to increase the output level difference in case that the output level difference is diminished due to the increase of the spatial frequency. Two examples of the relationships between the picture elements and the output levels are illustrated with the MTF correction amounts A1, A2, in FIG. 4.
In the optical image reading apparatus, it is desirable to make the output correction as for each picture element, regardless of the variation or pattern of the light and shade of the original image. However, according to the abovementioned MTF correction method, the MTF correction amount A1 becomes large when the output level difference between the peripheral picture elements is large as shown in FIG. 4a, while the MTF correction amount A2 becomes small when the output level difference between the peripheral picture elements is small as shown in FIG. 4b, resulting in the drawback that an ideal output correction cannot be always performed.
FIG. 5 are graphs each showing the MTF characteristic of a CCD (Charged Coupled Device) image sensor with respect to a light of the wavelength 550 nm. More particularly, the relationship between the MTF in the X direction (X-MTF) and the normalized spatial frequency as well as the spatial frequency without normalization is shown in FIG. 5a, while the relationship between the MTF in the Y direction (Y-MTF) and the normalized spatial frequency as well as the spatial frequency without normalization is shown in FIG. 5b. Here, the X direction is a direction along the array of the CCD sensor, while the Y direction is perpendicular to the X direction along the original plane.
In FIG. 5a, on one hand, when the normalized spatial frequency is 0.5, the MTF is 0.89. On the other hand, when the normalized spatial frequency is 1.0 i.e. at the Nyquist limit, the MTF is 0.68. This CCD MTF characteristic of FIG. 5a is listed in Table. 1.
The MTF characteristic of a lens is also listed in Table. 1. Namely the lens MTF is 0.79 when the normalized spatial frequency is 1.0, while it is 0.94 when the normalized spatial frequency is 0.5. Here, since a total MTF characteristic of an combinational optical system of such a CCD image sensor and a lens, is equal to their product, this total MTF characteristic is 0.54 when the normalized spatial frequency 1.0, while it is 0.84 when the normalized spatial frequency is 0.5, as also listed in Table. 1.
TABLE 1 ______________________________________ NORMALIZED CCD LENS TOTAL SPATIAL FREQUENCY MTF MTF MTF ______________________________________ 1.0 0.68 0.79 0.54 0.5 0.89 0.94 0.84 ______________________________________
In general, the MTF characteristic is regarded to be sufficient when the MTF is not less than 0.6, with respect to an ordinary optical image reading operation. However, as shown in Table 1, when the normalized spatial frequency is 1.0, the total MTF is as low as 0.54. That is to say, a sufficient output can not be obtained. As for the MTF in the Y direction (Y-MTF) shown in FIG. 5b, the situation is also similar.
Consequently, there is a problem that an image defect and an image destruction are caused upon encountering the Nyquist limit, i.e. the limit of the image resolution inherent to the sampling manner of the picture element, and thus an image input operation precisely corresponding to the original image is difficult to perform according to the above-mentioned signal image processing apparatus.